In the manufacture of integrated circuit chips, the use of tungsten (W), molybdenum (Mo) or other refractory metals for electrical contacts is well known. Typically, electrical contacts must be formed on exposed regions of a semiconductor substrate through openings in a patterned mask layer, typically silicon dioxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4) or another insulator. A persistent problem in the art has been how to selectively deposit tungsten on the surface of the semiconductor substrate without depositing tungsten on the mask layer. If tungsten deposits on the mask layer, it may form a short circuit between adjacent contacts, thereby rendering the device inoperable.
The need for selective deposition of materials other than refractory metals is also well known in the manufacture of integrated circuit chips. For example, nonrefractory metals often must be deposited in exposed regions of a substrate through openings in a patterned mask layer. Often, semiconductor materials such as silicon or germanium must be similarly deposited. Selective deposition of these semiconductor materials may require deposition of amorphous or polycrystalline semiconductor material. Alternatively, a monocrystalline deposition, often referred to as an epitaxial deposition, may be required. As is the case for refractory metals, a persistent problem in the art has been in how to deposit non-refractory metal, semiconductor or other material on the surface of the substrate without depositing the material on the mask layer.
Prior art techniques for depositing tungsten on the surface of a silicon substrate typically begin with a substitution reaction in which tungsten from tungsten hexafluoride (WF.sub.6) source gas in the presence of hydrogen substitutes for the silicon (Si) at the exposed surface of the semiconductor substrate according to the following reaction: EQU 2WF.sub.6 (gas)+3Si(substrate).fwdarw.2W(substrate)+3SiF.sub.4 (gas)(1)
This reaction is a truly selective one in that the tungsten hexafluoride will not react with a silicon dioxide or silicon nitride mask layer. However, this reaction is self-limiting in that after a few hundred .ANG.ngstroms of silicon is converted to W and the exposed silicon surface is completely sealed by W, no further substitution takes place. At this point, the hydrogen and tungsten hexafluoride gases interact to create a gas reduction reaction as follows: EQU WF.sub.6 (gas)+3H.sub.2 (gas).fwdarw.W(deposited)+6HF(gas) (2)
In this reaction W is deposited on the substrate in a nucleation type of process in which discrete sites of tungsten are deposited and then grow to form a continuous layer. It has been found that it is impossible to avoid nucleation centers from forming on the mask layer as well after a certain thickness, typically 2000 .ANG. or less, has been deposited. When depositing thick films of W (that is films thicker than about 2000 .ANG. or less), it has been found that tungsten nucleation on the mask surfaces causes a loss of selectivity in the tungsten deposition process. Eventually, a continuous layer of tungsten bridging the mask surface is formed, rendering the device inoperable.
Many techniques for eliminating or removing nucleation sites on the mask surfaces have been proposed so that selective deposition on the semiconductor surface may take place. Unfortunately, each of these techniques also produces a new set of problems for the tungsten deposition process. For example, U.S. Pat. No. 4,617,087 to Iyer et al discloses the use of NF.sub.3 and a plasma in the reaction chamber to create a simultaneous etching condition for the tungsten. The amount of NF.sub.3 and the plasma power coupled into the chamber are such as to ensure that the mask surface is kept clean at all times. Since the deposition rate on the exposed W surfaces is higher than on the mask surfaces, there will be net deposition on these areas despite the etching action. Unfortunately, the Iyer et al method requires the introduction of a new gas (NF.sub.3) into the process so that at least three simultaneous gas kinetic processes must be controlled and balanced. Moreover, a plasma needs to be struck and maintained in the reaction chamber, thereby greatly increasing the complexity of the process and of the system.
Other techniques for removing the nucleated refractory metal from the mask layer are disclosed in published European Patent Application No. 238,024 to Shioya et al (Sept. 23, 1987). Shioya discloses three techniques for removing the tungsten nucleation sites from the mask layer. The techniques are: (a) heating the substrate in hydrogen gas to produce hydrofluoric acid (HF) etchant from residual WF.sub.6 in the chamber; (b) wet etching in an HF solution; or (c) dry etching in nitrogen trifluoride (NF.sub.3) gas. Repeated deposition and removal steps are employed to deposit the required thickness of tungsten.
Unfortunately, the first Shioya technique is based on the hope that there will be residual WF.sub.6 within the reaction chamber or on the walls of the chamber. A stable and reproducible process cannot be designed around the presence of residual WF.sub.6 in the chamber. The second (wet etch) technique requires removal of the specimen from one reaction chamber and the use of a second reaction chamber for wet etch. Finally, the third technique (NF.sub.3 dry etch) introduces a new gas into the deposition process requiring the control and balance of three simultaneous gas kinetic processes as in the Iyer, et al. patent. It is apparent that those skilled in the art have sought, and have failed to find, an appropriate method for removing nucleation sites on a mask layer during a tungsten deposition process.
Attempts have also been made to deposit materials other than refractory metal on the surface of a silicon, or other monocrystalline substrate. Examples are silicon on silicon, germanium on germanium or silicon on sapphire. During such processes, nucleation sites form on the mask surfaces, thereby causing a loss of selectivity. An appropriate method for removing nucleation sites on a mask layer deposition of materials other than refractory metals has heretofore not been found.
Prior art techniques for depositing silicon or other semiconductor materials onto a substrate typically employ conventional deposition from a semiconductor-containing gas mixture. See for example U.S. Pat. No. 3,998,673 to Chow which discloses the use of silicon tetrachloride and hydrogen for selective silicon epitaxy. This technique typically results in deposition of unwanted discrete nucleation sites of silicon on the mask.
Many techniques for eliminating or removing silicon or semiconductor nucleation sites on the mask surfaces have been proposed. Unfortunately, as was the case for refractory metal deposition, each of these techniques also produces a new set of problems. For example, U.S. Pat. No. 4,497,683 to Cellar et al. discloses the use of SiCl.sub.4 +H.sub.2 for deposition followed by an HCl etch or an insitu etch using the byproducts of the silicon deposition reaction. Unfortunately, an HCl etch requires introduction of a new gas (HCl) into the process so that at least three simultaneous gas kinetic processes must be controlled and balanced. An in-situ etch relies on the proper byproducts of the silicon deposition reaction being present in the critical concentrations. A stable and reproducible process is difficult to design based on an in-situ byproduct etch.
Another technique for removing silicon nucleation sites is disclosed in U.S. Pat. No. 4,578,142 to Corby, Jr., et al. A two stage epitaxial silicon deposition and etch cycle is disclosed. During the first stage, silicon is deposited from a gas mixture which includes silicon source gas and a carrier gas. A silicon etching gas may also be included. In the second stage, silicon nucleation sites on the mask are etched in a mixture of silicon etching gas and a carrier gas. The stages are repeated to build up the required thickness of silicon. In one embodiment, dichlorosilane is used as the silicon source gas while hydrochloric acid is used as an etch gas during both the first and second stages, and hydrogen is also used as the carrier gas during both stages. Unfortunately, this process requires a simultaneous deposition and etch in the first stage, which is difficult to control in a repeatable manner. Moreover, this process requires control of three simultaneous reactive gas kinetics, i.e. dichlorosilane, hydrochloric acid and hydrogen.
As was the case for refractory metals, it is apparent that those skilled in the art have sought, and have failed to find, an appropriate method for removing nucleation sites on a mask layer during a semiconductor deposition or epitaxy process.